1. Field of the Invention
This invention relates to a multibeam scanning optical system and an image forming apparatus using the same, and particularly is suitable, for example, for a laser beam printer (LBP), a digital copying machine or the like using a plurality of light sources as a light source to achieve a high speed and high recording density.
2. Related Background Art
In an image forming apparatus using a multibeam scanning optical system applied to a laser beam printer, a digital copying machine or the like, a plurality of beams emitted from a plurality of different light emitting points (laser beam sources) are directed as a plurality of light spots to different positions separate from one another in a sub scanning direction on the surface of a recording medium such as a photosensitive member which is a surface to be scanned, by a common multibeam scanning optical system comprised of a light deflector such as a rotary polygon mirror and an fxcex8 lens or the like, and the surface of the recording medium is scanned at a time in a main scanning direction to thereby effect the recording of image information.
In such a conventional image forming apparatus, the distance in the sub scanning direction between adjacent spots scanned at a time, in other words, the scanning line pitch by the plurality of spots, must be adjusted with good accuracy.
Also, in recent image forming apparatuses, it is intended to make the resolution of an output image variable, and from such a point of view, it is desirable that the width of a plurality of scanning line pitches scanned at a time on the surface of a recording medium in the main scanning direction can be changed.
Further, recently, high recording density and a high-speed output have been more required, and it has become necessary to increase the number of the light emitting points of a multibeam scanning optical system in an image forming apparatus to 2, 3, 4, . . . .
Heretofore, to adjust or change the scanning line pitch by a plurality of spots, a light source unit comprised of a plurality of light sources and a beam combining prism or the like has been rotated about the optical axis of an optical system in a plane perpendicular to the optical axis. A multibeam scanning apparatus of this kind is proposed, for example, in Japanese Patent Application Laid-Open No. 7-181412.
The adjusting method for the scanning line pitch in the above-mentioned publication, however, is of a construction in which the light source unit comprised of a plurality of light sources and a beam combining prism or the like is rotated about the optical axis of the optical system in the plane perpendicular to the optical axis and therefore, suffers from the problem that the scanning line pitch is liable to vary for mechanical vibration or the like. Also, the laser beam sources such as semiconductor lasers are fixed to a movable portion, and this also leads to the problem that stability to environmental fluctuations (e.g. changes in temperature, etc.) lacks and the pitch interval is liable to change with the environmental fluctuations.
It is an object of the present invention to provide a multibeam scanning optical system witch is strong against mechanical vibration or the like and suffers little from pitch fluctuation and which is excellent in environmental stability, and an image forming apparatus using the same.
It is a further object of the present invention to provide a multibeam scanning optical system in which at least one light source is comprised of a monolithic multibeam laser having a plurality of light emitting points to thereby cope with the requirement for a higher speed and a higher quality of image, and an image forming apparatus using the same.
In one aspect of the invention, a multibeam scanning optical system comprises a light source unit having a light source and a plurality of beam converting means having a first optical system for converting a beam emitted from said light source into a divergent beam or a convergent beam, beam combining means for combining a plurality of divergent beams or convergent beams emerging from said light source unit and causing them to emerge substantially in the same direction, a second optical system for causing the plurality of divergent beams or convergent beams combined by said beam combining means to be imaged as a linear image long in the main scanning direction, deflecting means having a deflecting surface at or near the imaging position of said second optical system, and for reflecting and deflecting the plurality of incident divergent beams or convergent beams in the main scanning direction, and a third optical system for causing the plurality of divergent beams or convergent beams reflected and deflected by said deflecting means to be imaged on a surface to be scanned, and adjusting means for adjusting the relative position of said plurality of divergent beams or convergent beams in the sub scanning direction to thereby adjust the relative imaging position of said plurality of beams in the sub scanning direction on the surface to be scanned.
In further aspect of the multibeam scanning optical system, said beam combining means has a beam combining prism.
In further aspect of the multibeam scanning optical system, said adjusting means rotates said beam combining prism about an axis lying in the main scanning cross section and substantially parallel to the surface to be scanned.
In further aspect of the multibeam scanning optical system, said adjusting means moves at least one of said plurality of beam converting means in the sub scanning direction.
In further aspect of the multibeam scanning optical system, the distances from the principal point of said third optical system on the deflecting means side in the main scanning cross section to the natural converging points of said plurality of divergent beams or convergent beams are equal to each other.
In further aspect of the multibeam scanning optical system, a light source constituting an element of at least one of said plurality of beam converting means is comprised of a monolithic multibeam laser having a plurality of light emitting points.
In further aspect of the multibeam scanning optical system, the adjustment of the relative imaging position of the plurality of beams emitted from the plurality of light emitting points of said monolithic multibeam laser in the sub scanning direction on said surface to be scanned is effected by rotating said monolithic multibeam laser about an optical axis.
In further aspect of the multibeam optical scanning apparatus, the multibeam scanning optical system is contained in a housing.
In another aspect of the invention, an image forming apparatus comprises the foregoing multibeam optical scanning apparatus, a photosensitive member disposed on said surface to be scanned, a developing device for developing as a toner image an electrostatic latent image formed on said photosensitive member by a beam scanned by said multibeam optical scanning apparatus, a transferring device for transferring the developed toner image to a transfer material, and a fixing device for fixing the transferred toner image on the transfer material.
In another aspect of the invention, an image forming apparatus comprises the foregoing multibeam optical scanning apparatus and a printer controller for converting code data input from an external device into an image signal and inputting it to said optical scanning apparatus.